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May 5

Homebrew sunscreen

I want to make my own homebrew version of Ocean Potion Dab-On brand sunscreen because $4+ for 0.6 ounces is a little pricy for what I would prefer to be able to apply more liberally if I could use an applicator cloth instead of the magic marker format that they sell it in, and because I hate greasy creams which invariably clog my pores, and because sprays can be rude (especially where food or drinks are being consumed) and smelly, and aerosol cans are hard to take on planes. So:

(Questions 1-4) Where can I purchase Homosalate (15%), Octocrylene (7%), Octyl salicylate (5%), and Avobenzone (3%)?

(Questions 5 & 6) Can the remaining 70% be isopropyl alcohol and glycerin, and if so in what proportion?

(Question 7) Do I need to copy any of the other inactive ingredients as stabilizers, preservatives, detoxificants, etc?

Thank you Wikipedians in advance for helping to keep my skin healthy wholesale. 209.210.168.146 (talk) 01:47, 5 May 2015 (UTC)[reply]

The alcohol is going to smell strongly and dry out your skin, while the glycerin will feel greasy. As for saving money, you'd need to produce your own in large quantities to save money per ounce. I suggest you just keep looking for commercial products you like, at a better price. Dollar stores can be a good place to start (some of their products are inferior, while others were just overstocked, etc.). StuRat (talk) 02:03, 5 May 2015 (UTC)[reply]

No more than aftershave, right? I'm pretty sure that the manufacturer in Cocoa, Florida has been using patents to corner the market in the simple, aqueous solution application format, so there might not be any competing products. It looks like my biggest concern will be keeping it in an opaque bottle, per Avobenzone#History, although there is a whole lot in that article section which is way over my head chemically. How do I find suppliers for the active ingredients? 209.210.168.146 (talk) 02:12, 5 May 2015 (UTC)[reply]

Well, you'd want a chemical supply company, but they might sell many of those by the kg or liter, for hundreds of dollars each. So, you'd need to make hundreds or thousands of bottles of sunscreen to make them cheaper. And note that you don't know the order the ingredients are added, the temperature at which they are mixed, etc. StuRat (talk) 02:17, 5 May 2015 (UTC)[reply]

Yes, but how to find such suppliers? I have my own proven method for testing sunscreen, which involves shining an ultraviolet LED through a measured quantity of sunscreen sandwiched in plastic wrap "slides", and measuring the amount of fluorescence produced (e.g. in tonic water or on bleached pulp paper) on the opposite side of the slide. I have used this technique to identify fake sunscreens sold overseas, and am confident that I can experiment with small batches and patches on skin. I don't think any of the ingredients are very sensitive to temperature, as long as they are in a polar solvent, but ... finding the suppliers is the next step at this point. 209.210.168.146 (talk) 02:31, 5 May 2015 (UTC)[reply]

Have you tried Googling "buy X", where X is one of the ingredients ? StuRat (talk) 02:35, 5 May 2015 (UTC)[reply]

Not until you suggested it. Most of the time when I type the name of something I want to buy, Google just assumes I want to buy it. That's exactly the answer I wanted. Five out of five, would recommend Wikipedia's Reference Desk to friends. Thank you. 209.210.168.146 (talk) 02:46, 5 May 2015 (UTC)[reply]

Sigma-Aldrich, one the largest chemical suppliers, sells all four of those [1], but typical prices are ~$50 in 1 gram quantities, or ~$300 in 10 gram quantities. You can probably get better prices by Googling around. I would suggest queries like "homosalate price" / "homosalate order" / "homosalate quote" Dragons flight (talk) 02:43, 5 May 2015 (UTC)[reply]

Six out of five. Thank you. 209.210.168.146 (talk) 02:46, 5 May 2015 (UTC)[reply]

Incidentally, if you Google those active ingredients together, you can find some similar products that might be worth exploring before spending the time and money to brew your own. For example, Neutrogena Dry Touch sunscreen has the same four active ingredients (plus one additional active ingredient), and only costs $10 for a 3 oz tube. Dragons flight (talk) 02:57, 5 May 2015 (UTC)[reply]

Seven out of five. Then again, compare [2] to [3]. I suspect I want to stick with the more well reviewed formulation to start with. Now I wonder what kind of liability insurance I would most wisely need to even start experimenting on myself. 209.210.168.146 (talk) 03:16, 5 May 2015 (UTC)[reply]

That would be determined by how much you intend to sue yourself for when it all goes wrong. Dbfirs 07:33, 5 May 2015 (UTC)[reply]

If you want to go with the "more well reviewed formulation", you should use the same source of reviews. Neutrogena at Amazon has ~300 reviews and an average rating of 4.4 stars. Ocean potion at your link has 23 reviews and a rating of 4.3 stars. --Stephan Schulz (talk) 08:11, 5 May 2015 (UTC)[reply]

Why is the eyes cavity called "orbital" cavity?

149.88.6.66 (talk) 05:12, 5 May 2015 (UTC)[reply]

Because it's circular. Comes from the Latin word orbis meaning ring. The use dates to the mid 16th century, as Merriam Webster tells it, "from Latin orbita 'course, track' (in medieval Latin 'eye socket'), feminine of orbitus 'circular,' from orbis 'ring.' - Nunh-huh 05:18, 5 May 2015 (UTC)[reply]

And while words beginning with "orb-" originally referred to a circle, "orb" also is now often used for spherical objects, such as the eyeballs.[4] ←Baseball Bugs ^{What's up, Doc?} carrots→ 05:51, 5 May 2015 (UTC)[reply]

Hairy day!

This question has been removed. Per the reference desk guidelines, the reference desk is not an appropriate place to request medical, legal or other professional advice, including any kind of medical diagnosis, prognosis, or treatment recommendations. For such advice, please see a qualified professional. If you don't believe this is such a request, please explain what you meant to ask, either here or on the Reference Desk's talk page.

Talk page discussion about the deletion is here: [5]. StuRat (talk) 01:27, 9 May 2015 (UTC)[reply]

Low temp Rankine cycle generators?

I am trying to determine if there's any existing closed-cycle technology for recovering mechanical or electric power from small, low-heat thermal sources, using low-boiling-point liquids like ammonia or ethyl alcohol (non-burning) to run a small (perhaps less than 25 kw) electric generator. This is not discussed in the Rankine cycle article.

This is basically the reverse of an air conditioner: heat the liquid to a gas, run it through a turbine or piston pump, and turn a generator. The exhaust gas is cooled and returns to a liquid state through a small liquid pump, before running through the boiling cycle again. -- DMahalko (talk) 12:26, 5 May 2015 (UTC)[reply]

Not exactly what you describe, but have you looked at Stirling engine?--Phil Holmes (talk) 12:51, 5 May 2015 (UTC)[reply]

Look at Organic Rankine Cycle 118.137.194.159 (talk) 14:36, 5 May 2015 (UTC)[reply]

EmDrive: How sure are we that sound cannot travel in a vacuum?

(Previous discussion: [6])

I recently saw a news item about the EmDrive succeeding in a vacuum chamber test - which I see has been added to our article - at [7]. Now that article seems pretty ridiculous (it's sourced to a forum), but some of the people on the forum seem to know physics, and it looks like they're discussing figures from a paper, so I'm wondering if a reliable source can be pulled out somewhere.

Now the theory of the EmDrive as presented by the company seems ridiculously incomplete and uninformative.[8] The usual explanation for this online is that it is pseudoscience masquerading as science, but with more successful testing I have to consider the most unlikely but most fascinating possibility that it could be science masquerading as pseudoscience, i.e. that the inventors simply don't want to explain how it works intelligibly until they finish all their patents.

Anyway, our take on the principle is that it is a Quantum vacuum plasma thruster (the article is practically a fork of EmDrive, and bigger). But my feeling is that speaking of "virtual particles" is generally confusing; as with the Casimir effect more sense can be made of a phenomenon when they are not used.

Is it correct to say that a QVPT, if possible, is transmitting vibrations, but no real particles, with some real velocity?

I'm not sure how you calculate wave momentum of ordinary sound in air or water. I see some discussion at [9] but I could use something more satisfying. Is it possible to predict, in principle, whether transmission of solitons through hard vacuum could provide more delta momentum per delta energy than a photon drive?

If vacuum energy exists, does that imply that vacuum is a medium with mass that is capable of carrying sound? Would the relationship of the vacuum energy to the mass-energy of air give a sense of the impedance mismatch between air and vacuum?

How exacting of a test has thus far been made of the inability of sound to jump a vacuum gap?

Wnt (talk) 14:32, 5 May 2015 (UTC)[reply]

Wnt, this is not so much a question of certainty about experimental evidence, as it is purely a question of the definition of sound, and the definition of a vacuum. Sound is defined as the vibration of matter in a medium. A vacuum is defined as a region in which there is total absence of any matter. From these definitions, we arrive at contradiction in terms: there is no sound propagation in a vacuum.

If you are interested in scenarios that are, by some stretch of the imagination, analogous to sound propagation in a vacuum, then you can talk about things like plasma waves that can propagate in very low density gas or plasma; you can talk about electromagnetic waves; and so on. For these kinds of questions, you can begin by reading about magnetoacoustic waves, Alfvén waves, Langmuir waves, and so on. If you want more information, the canonical reference book is Bittencourt's Fundamentals of Plasma Physics. These waves are propagations of pressure fronts, and they are very much like an acoustic wave, but they can persist in very low density plasma because the energy is conveyed by long-range dynamic electromagnetic forces (rather than by short-range, electrostatic forces that we conventionally represent as intermolecular collisions).

Last week, I had some time away, and I picked up a copy of Lettres Provinciales and the Scientific Thoughts of Blaise Pascal. (The library here stocks Great Books of the Western World, a series published by Encyclopedia Brittanica many years ago - pick up a copy next time you find one!) Your question resonates so strongly with this reading material: it's like you want to twiddle over the definitions, because you can't quite figure out how to say what you mean when you say a vacuum has nothing in it and yet it has something in it. I won't even try to refute that: M. Pascal eloquently refuted that position nearly four hundred years ago. Nimur (talk) 14:46, 5 May 2015 (UTC)[reply]

The modern vacuum does have stuff in it, such as a nonzero Higgs field. The Higgs field can oscillate around its nonzero average value, and the oscillations are called Higgs bosons (and longitudinal W and Z bosons). If the dark energy is likewise a field, it can vibrate in the same way, and that would be a particle. The fields that have an expected value of zero in the vacuum can also oscillate around zero, and those oscillations are particles. A device that uses vacuum oscillations for propulsion is a rocket. I suppose that there is some (very tiny) transmission of sound across a vacuum by phonons coupling to vacuum oscillations, but this would not be new physics, just a very inefficient photon drive. -- BenRG (talk) 18:42, 5 May 2015 (UTC)[reply]

Also, just in case it is not abundantly obvious: NASASPACEFLIGHT.com is not affiliated with NASA. Official government information is available at NASA's website, http://www.nasa.gov and official information about exploratory research in ion propulsion is hosted at Glenn Research Center's technology web page. The limited information on the "EMDrive" probably refers to this specific report, Anomalous Thrust..., published last year (and later thr subject of a popular-science media flurry). If you read that report, you'll see that (a) NASA did test a device, although they did not work on it; and (b) there has not been extensive follow-up research on this topic. Nimur (talk) 16:37, 5 May 2015 (UTC)[reply]

You're right that NASASPACEFLIGHT.com is not affliated with NASA, but if you read their article, it seems that someone from NASA Eagleworks has posted in a forum there and shared newer information and also clarified some aspects of their research. Of course such unpublished personal communication should be taken with a grain of salt (although not that different from a blog), but it's also clearly not simply a media flurry from 2014 information (even if most of the core stuff was already published in 2014 and most of the media flurry doesn't relate to the new information). Edit: Actually I think the vacuum stuff wasn't revealed until this year [10] [11]. See also Harold G. White (NASA). So yeah, even more a case of not 2014 info even if it was only revealed by forum posts. Nil Einne (talk) 18:16, 5 May 2015 (UTC)[reply]

If I recall correctly, the "Anomolous thrust..." paper reported on testing the device in an evacuation chamber that was not actually evacuated during the test procuedure. Possibly relevant to OPs questions. SemanticMantis (talk) 18:53, 5 May 2015 (UTC)[reply]

The classic experiment is to put an alarm clock into a bell jar and gradually pump out all of the air. The sound from the ringing bell gets quieter and quieter as the air is removed. Does the sound ever COMPLETELY go away? No, it doesn't...but that's not because sound is able to travel through a vacuum - that's impossible (as Nimur explains) by the very definition of the term. The sound is still there because a total, perfect vacuum is utterly impossible to achieve (and in this case, sound is also transferred through the bottom of the bell jar upon which the clock is resting).

Large large-scale sound waves have been detected as they propagate through "the vacuum of interstellar space" - because that's not a vacuum - there are traces of interstellar gasses. But the amount of energy that can be transferred gets less and less as the density of the gasses decreases. Put another was, the sound gets quieter and quieter, but it's still there because there are still atoms floating around and colliding once in a great while - which is the mechanism by which sound is propagated.

A very loud sound - such as an explosion in a near vacuum is still going to transmit enough energy to be detectable - and even in interstellar space, the 'sound' of a supernova going off transmits sound through near vacuum with enough energy that we're able to measure it.

So in a purely theoretical sense, sound can't exist in a vaccuum - but in a practical sense, this is all about how much sound? and how imperfect the vacuum?, whether you consider one atom moving along and hitting another atom to be "sound propagation" or "transfer of momentum" is just a matter of which words you choose - in the end, at the physical level, they are the exact same thing. Words like "sound" and "temperature" which basically describe bulk properties of large quantities of matter, cease to have much meaning as the quantities of matter decrease. Nothing physically special happens - it's just that our relatively crude terminology doesn't fully explain what's going on.

SteveBaker (talk) 17:46, 5 May 2015 (UTC)[reply]

A good concept which grasps what SteveBaker is getting at is that of the Emergent property. Sound and temperature are emergent properties of molecular motion: they don't make sense without matter, and they don't even make sense on the individual molecule level. They only make sense as properties of the bulk medium; of lots and lots of molecules interacting with each other. Thus, the property is nonsensical on the single molecule level, but emerges from the interactions of many of them. --Jayron 32 19:22, 5 May 2015 (UTC)[reply]

@SteveBaker: There have been many great answers here, including yours and Nimur's, but the part about how sound can't travel through a vacuum by definition seems a little too glib to me. You can picture doing the bell jar experiment, extrapolating the curve of the sound detected at a microphone as you pump out the vacuum... and finding it doesn't intersect the origin. In which case for all intents and purposes you'd have to admit that 'sound' crossed the vacuum, and the definition couldn't make that go away. So the question is: how well has this been researched, and what edge cases have been explored? Can a soliton use quantum tunneling to cross a small vacuum gap in a solid, or a barrier of impedance-mismatched material? Is there some kind of near field effect for solitons that would allow such gap-crossing?

@Nimur: You know vastly more about them than I do, but a plasma wave is still basically a vibration of particles (with added electromagnetic components). Is it plausible that a "virtual quantum fluctuation of the vacuum, when annihilating its "real" antiparticle to balance the books, could do so in the same way whether it is interacting with a sound wave or a plasma wave? (i.e. whether the device being tested is in air or in vacuum)

I should add that part of the fun here is that if some kind of vibration could be sent through a vacuum, not only would it be a (sort of) "reactionless" drive, but perhaps also a method of interstellar communication? Wnt (talk) 20:24, 5 May 2015 (UTC)[reply]

No - you completely failed to understand. If I plot a graph of transmitted sound energy versus air pressure - then when the air pressure hits zero, so does the sound intensity. It's a PERFECT intersection at (0,0). No sound whatever crosses a perfect vacuum.

What you're missing is that there is not - nor could ever be - a perfect vacuum. No matter how good your vacuum pump, no matter how perfect your seals, there will always be outgassing from the material that the surrounding vessel is made from - and the time to remove the last molecules of air gets exponentially longer as the pump draws the vacuum down - so it'll never remove it all. Also, all materials - even glass and steel, evaporate material off of their surfaces to pollute your ideal vaccum. So there is no such thing a perfect vacuum - not anywhere in the universe...there is ALWAYS something there - albeit in very low quantities.

So, if I turn on my vacuum pump and plot a graph of sound energy versus TIME, then the curve never intersects the zero sound energy line - but that's not because sound can jump a vacuum - it's because if I also plot air pressure against time, that never hits zero either. So you're simply failing to comprehend the actual science/math involved here.

But in the realm of pure theory where true/perfect vacuums can be thought about...it is utterly impossible for sound to jump across a perfect vacuum because the definition of the word 'sound' is the vibration of atoms - and the definition of 'vacuum' is the absence of atoms. Since there are no atoms to vibrate - so there cannot be any sound because that's what sound is. If something else is vibrating in the vacuum (like the electromagnetic field) then there is light, or radio - but it ain't sound.

It's like saying "In a vast treeless desert without any aardvarks, do aardvarks climb trees?" - I don't have to know anything about the climbing ability of Orycteropodidae to know that in this place, aardvarks don't climb trees. Similarly, you don't need to know any physics to know that sound can't cross a vacuum - you just need to crack open a dictionary and read the definitions of the words "sound" and "vacuum".

SteveBaker (talk) 20:51, 5 May 2015 (UTC)[reply]

I think you're talking a little bit at cross-purposes with the Wnt. He's not talking about a vacuum that fails to be perfect because there are a few atoms you didn't manage to clear out. He's talking about a quantum vacuum, which is not really empty even if you do manage to clear out all the (non-virtual) atoms.

I am also not entirely persuaded that sound has to be transmitted by atoms; that seems a fairly arbitrary restriction. I would think that any pressure wave mediated by particles bouncing off each other would count as "sound". So Wnt's point is, why can't those particles be virtual? I don't know whether that's possible or not, but I haven't seen that the point has been really taken seriously in the responses. --Trovatore (talk) 21:08, 5 May 2015 (UTC)[reply]

Trovatore, your point (and Wnt's question, for that matter) are fair... but it's still ultimately a definition. To phrase this answer simply, in direct response to Trovatore's most recent query, yes, sound must be transferred by atoms, because if it were anything else, it would not be called "sound." Sound is a longitudinal wave that propagates as acoustic compression of atoms. There are lots of things that are similar to sound: there are vibrational fronts that are non-longitudinal waves - like the shear wave in a material or in an earthquake. There are longitudinal waves that are not movement of atoms: like the electron oscillation propagations in certain types of low density plasmas or in the electron gas model of semiconductors. When physicists reason abstractly about the mathematics to describe such phenomena, they may find many similarities to conventional sound. But these phenomena are not sound! Sound is the word we use to describe wave propagation of energy, by way of movement of atoms as a longitudinal compression, mediated by intermolecular collision.

It is very interesting to wonder if other exotic physical interactions - like Higgs fields, dark energy, and so forth - could act as an elastic or compressive medium, or if they could mediate a longitudinal propagating wave front. That is an interesting question independent of the name we call it by - but it is confusing (and I daresay incorrect) to call that a question about sound. Regrettably, Wnt, I am not actually an expert in exotic physics: I don't want to mislead you by pretending I know that answer. My recommendation, though, is to study conventional physics "to completion," before you attempt exotic physics. Many complex details of mathematical physics and wave mechanics are best understood when you thoroughly understand how those descriptions of "exotic" conditions differ from similar mathematical constructs that can describe "normal" conditions.

(Please note, in a few cases it is appropriate to replace my usage of "atom" with "molecule" - for example, in common conditions, sound travels in air mediated by intermolecular collisions of mostly Nitrogen molecules, ... under water, sound travels by way of collisions between mostly H₂O molecules... but you get the idea that these are interactions among conventional matter).

Nimur (talk) 21:47, 5 May 2015 (UTC)[reply]

So the question about the word "sound" is more a language question than a science one, but I am still not convinced. Are you saying that (say) pressure waves passing through a neutron star are not "sound", because they are propagated by degenerate matter rather than atomic matter? I doubt that that is standard usage. --Trovatore (talk) 22:09, 5 May 2015 (UTC)[reply]

Indeed, a neutron star might have pressure oscillations, but they are not "sound." Contrast this with, say, helioseismology, where some of the oscillations are sound, other pressure fronts are magnetosonic waves, still others are purely electromagnetic, or mediated by nuclear forces, and so on. I will not be surprised if you can find some research where the analogous behavior in a neutron star or pulsar is described as "sound," but it is my position that this is abuse of notation, either deliberate or accidental. Consider, for example, many notable instances of "sounds of space" in which plasma waves are called "sound", even though the frequencies are all wrong, and the transducer that detects the wave is electromagnetic in operation. (NASA's narrator directly addresses the issue in the video related to that story). This is "abuse of terminology" to make a point, or to pique interest, but it is not technically accurate. Nimur (talk) 23:33, 5 May 2015 (UTC)[reply]

Isn't it interesting how sound is an electromagnetic interaction at it's basest level? When the atoms, molecules or what have you interact, they are mutually repulsed by each other's EM fields. The trouble is that the field of an single uncharged atom, or molecule diminishes very quickly with distance, so this is a short range interaction. Get too far apart as in a hard vacuum and the interaction becomes so weak, that it is practically non-existent. Plasmic Physics (talk) 22:20, 5 May 2015 (UTC)[reply]

Yes, this is true; but it is the quantitative nature of the interaction that matters: can the interaction be modeled as a compressive restoring force or elastic collision? If yes, it can convey a longitudinal compression wave. If no, it cannot; and although the interaction may permit wave propagation, that wave is not acoustic. Nimur (talk) 23:33, 5 May 2015 (UTC)[reply]

One way in which I'd think of measuring "sound" crossing a vacuum is if you have a very loud speaker on an object floating in a variable degree of vacuum or gas/vapor, and you attempt to measure sound with a microphone attached to a sphere that surrounds it. In this experiment, the question would be if the intercept of the energy detected at the microphone vs. the amount of gas/vapor present truly passes through the origin. There may be much better ways to do this experiment, but an experiment it is. Of course, in this case we'd have to suppose that the sound has to convert into something that can cross a vacuum, whether that be quantum fluctuations or something else. To give a more real (but less interesting) example of a mechanism, I suppose that if you made the inner sphere and outer sphere out of something piezoelectric, you should be able to detect considerable transmission via the generation of an electric field at the source that causes a deformation in the surrounding sphere.

I'll admit it sounds like I've been using "sound" in a sensu very largo to include anything that can be converted back and forth to sound, and I welcome suggestions for a more general term.

Of course, what I'm really interested in for the EmDrive is if there might be some way to extract the fluctuation entirely out of the matter it fluctuates in, and send it across hard vacuum, and if that extracted essence could be carrying with it a very small amount of energy and yet a large amount of momentum. Actually, the ultimate target question is whether these fluctuations might be used as a method of interstellar communication by alien civilizations who expect anyone capable of responding usefully to have an EmDrive. :) Wnt (talk) 01:17, 6 May 2015 (UTC)[reply]

You haven't listened to a word from anyone, no form of sound can cross a vacuum; and the total energy delivered by a sound wave has to pass through the origin by definition. Sound cannot be converted be 'converted' to something that can cross a vacuum, only the energy that it caries can be transduced. You can redefine sound to mean whatever you like, but that does not make it so, and neither can we sensibly answer questions based on false premises. Plasmic Physics (talk) 11:33, 6 May 2015 (UTC)[reply]

In fact there is experimental evidence that sound, in the sense of vibration, can "tunnel" a short distance across a vacuum. Googling for "vacuum phonon" will get you into the literature -- phonons are basically quanta of sound. See http://physicsworld.com/cws/article/news/2010/oct/14/phonons-tunnel-across-the-vacuum, a news story from 2010, for more information. Looie496 (talk) 12:44, 6 May 2015 (UTC)[reply]

There have been a lot of great answers here - I realize I'm asking for a lot, and intentionally abusing the bounds of science. This result of phonons crossing a vacuum is at once mundane (to my untrained eye it seems closely related to the piezoelectric idea I mentioned above), and yet intriguing when it is described as phonons crossing a classically forbidden region. After all, we know that electrons can tunnel through a classically forbidden region between two conductors, and yet, when enough energy is put into the system, the transmission can manifest itself as a whole new creature, a free electron in space. So I'll hold out hope for the concept of a "free phonon" that is a vibration without matter to vibrate, just some kind of vacuum fluctuation. And indeed, that search phrase witched up this intriguing-sounding patent (news coverage, denunciation by physicists)- a "phonon maser" is used as the drive for a ship which, intriguingly enough, is also said to bend spacetime somehow. I think I'm starting to suspect the Chinese are so reticent about the theory because they're violating this guy's patent. :) Wnt (talk) 22:42, 6 May 2015 (UTC)[reply]

Fraction of mass in galaxies

I'm having trouble finding a source for this. Roughly what fraction of the mass of the universe is associated with galaxies, rather than intergalactic space? My back of the envelope calculation using "typical" galaxy masses and separation distances suggested that only 3% of all mass in the universe is associated with galaxies, but I'd like to see a more rigorous discussion. Dragons flight (talk) 17:41, 5 May 2015 (UTC)[reply]

Regular matter or also dark matter? --Stephan Schulz (talk) 17:42, 5 May 2015 (UTC)[reply]

Both together. Dragons flight (talk) 17:45, 5 May 2015 (UTC)[reply]

Does Observable universe#Mass of ordinary matter help any? --Jayron 32 19:17, 5 May 2015 (UTC)[reply]

Thanks. That led to a paper that mostly answered my question, though it only addressed baryons in galaxies and not also dark matter. Dragons flight (talk) 13:40, 6 May 2015 (UTC)[reply]

I think part of the problem with finding reliable numbers for dark matter is that, basically, dark matter is still code for "damned if we know" with regards to physical cosmology. Dark matter and its cousin dark energy are the way in which cosmologists "hand wave" off discrepencies between their observations of the universe, and the equations which should predict how the universe behaves. The estimates of the amount of each are dependent a LOT on which assumptions one makes about their characteristics, distribution, behavior, etc, most of which is highly speculative. I'm not saying you won't be able to find some numbers, I'm just saying those numbers are likely to vary wildly depending on which set of assumptions you are working with. --Jayron 32 13:59, 6 May 2015 (UTC)[reply]

Why right triangles are so important to describe lots of natural phenomena?

What made the right triangle the foundation of trigonometry and therefore the foundation for the description of lots of phenomena? Couldn't science just develop a kind of trigonometry bases in 60-60-60 triangles? Or based in 100-40-40 triangles? --Llaanngg (talk) 19:02, 5 May 2015 (UTC)[reply]

(edit conflict) They could have, but right angles have interesting properties. Orthogonality is an important principle when dealing with all sorts of physical phenomena, from wave interference to the electronic structure of atoms to projectile motion. It also allows for convenient work with Cartesian coordinates because the axes in such coordinate systems are set at right angles to each other; that describes a fairly natural way humans think of the world in Three-dimensional space, insofar as a "dimension" is defined as any measurement orthogonal to any other measurement. --Jayron 32 19:15, 5 May 2015 (UTC)[reply]

(edit conflict) A right angle is a geometric representation of orthogonality in the plane. Orthogonality is a very important, deeply fundamental concept in advanced mathematics. Constructing triangles, and relations among triangles, around an orthogonal coordinate system dramatically simplifies the presentation of a lot of mathematics. It is no coincidence, for example, that sine and cosine are orthogonal functions. You can write relations about any other constructed triangle, but the selection of an orthogonal basis set means that much complexity is eliminated.

If you are already comfortable with linear algebra, review our article on linear independence, particularly the section on vectors in R². If this type of mathematical notation is unfamiliar, a good book to start with is A Transition to Advanced Mathematics, which will help you understand some of the methods to make generalizations from elementary geometry into fundamental principles of mathematics.

Nimur (talk) 19:16, 5 May 2015 (UTC)[reply]

Wouldn't the simple lay answer be that we live in a 3D universe, and that motion in one direct at 90 degrees from another is totally independent, but that motion at 60 degrees is not fundamentally independent, and can be resolved into to separate motions in two directions. μηδείς (talk) 00:38, 6 May 2015 (UTC)[reply]

That's basically what orthogonality means... --Jayron 32 02:23, 6 May 2015 (UTC)[reply]

Not exactly, at least if we are talking about linear independence, since any two sides of any triangle would suffice for that. Orthogonality buys us much more "intuitive" and mathematical convenience in Euclidean space as you and Nimur have already explained. Abecedare (talk) 03:10, 6 May 2015 (UTC)[reply]

I was suggesting that my wording was more lay-friendly, not suggesting that I was unfamiliar with the concept of orthogonality. It's not correct that any sides of any triangle will do for the purpose, look at the second image in the linear independence article which shows acute angles as not meeting the criteria. μηδείς (talk) 18:25, 6 May 2015 (UTC)[reply]

So much easier with right angles. We get to use Pythagorean_theorem an all' right away :) Star Lord - 星王 (talk) 05:43, 6 May 2015 (UTC)[reply]

I'd second that. The maths gets much easier with right-angled triangles. So it's easier to make that the base of further analysis. In many cases you could probably do the same with 60-60-60 triangles, but why do it the messy way if it can be done easily. It's quite a general principle in math to deal with the easiest case first, understand that thoroughly and then use those results to work out the general situation. BTW, orthogonality is a high level concept. You can define it in many different ways. The default (and somewhat leader to the easier maths) is to have it co-incide with 90 deg. Again, other choices are possible, but it gets messy. 86.189.252.112 (talk) 22:10, 6 May 2015 (UTC)[reply]

Is Yi qi the first fully Chinese species name?

I've seen species names that contain all sorts of languages mixed together with Latin, but this one is the first one I've seen that's all Chinese. — Sebastian 23:19, 5 May 2015 (UTC)[reply]

I suspect Chinese scientists have been using non-Latin scientific names for thousands of years. Who is the canonical arbiter of a species' "official" name?

Perhaps if you rephrase this question to ask whether any major Western zoology or biology journal has previously accepted publications for a non-Latin binomial specieis identifier, ... the problem becomes more tractable.

Nimur (talk) 00:01, 6 May 2015 (UTC)[reply]

The idea of a Latin binomial began with Carolus Linnaeus, and was a radical innovation and step forward in biological taxonomy. There's nothing essential about the Latin part, although it is still traditional. It's simply false to imply that any other culture had this conceptual advance, especially for "thousands of years". It makes a joke of political correctness. What all cultures have had is common names, including all the Europeans. Nevertheless, you won't get a PhD anywhere if you insist on calling a panda anything other than Ailuropoda melanoleuca in a taxonomic setting. μηδείς (talk) 00:28, 6 May 2015 (UTC)[reply]

With due respect, I believe you are mistaken. Περὶ ζῴων γενέσεως used Greek scientific names when published a few thousand years ago. About a thousand years ago, those animal names were translated into Latin by Arab scholars and republished as كتاب الحيوان. Latin was the common language of scholarship for hundreds of years before Carl Linnaeus was born; and before that, Greek, Syriac, Persian, and other languages were used by scientists. Elsewhere science existed; China's Zhou Empire had zoological gardens before the Roman era - before Latin even existed. Linnaeus was hardly the first scientist to categorize wildlife in Latin, and he was hardly the first to be systematic about it.

Pliny wrote Naturalis Historia in Latin; Gesner wrote Historiae animalium in Latin. All of these were systematic scientific taxonomies, and some even used binomial nomenclature centuries before Linnaeus. Linnaeus is not the father of taxonomy, nor of scientific Latin; he is simply the father of Linnaean taxonomy, and as we now use many other methods to classify species, that taxonomy is hardly "current" scientific practice. It's just another historical work - one of many in a long line of improvements, translations, and missteps. I don't think there is any good reason to assume that Latin is a preferred language of science: it had a place in a certain part of the world for a certain duration of history, but it's rather small-minded to assume that Latin defined zoology - particularly now that so few people read it! Nimur (talk) 02:11, 6 May 2015 (UTC)[reply]

The fact that Greek and Roman authors wrote in Greek and Latin is hardly surprising, is it? It's also a historical accident that those languages were used in the early modern era in Western Europe, and entirely irrelevant to the scientific concept of binomial nomenclature. The notion that Latin makes it scientific is your own straw man, Latin was simply the lingua franca of naturalists at that time; there's no point in your demolishing what hasn't been asserted. Prior to Linnaeus there was no systematic use of binomials in an explicit Genus-species system. Read binomial nomenclature:

Prior to the adoption of the modern binomial system of naming species, a scientific name consisted of a generic name combined with a specific name that was from one to several words long. Together they formed a system of polynomial nomenclature.[2] These names had two separate functions. First, to designate or label the species, and second, to be a diagnosis or description; however these two goals were eventually found to be incompatible.[3] In a simple genus, containing only two species, it was easy to tell them apart with a one-word genus and a one-word specific name; but as more species were discovered the names necessarily became longer and unwieldy, for instance Plantago foliis ovato-lanceolatus pubescentibus, spica cylindrica, scapo tereti (Plantain with pubescent ovate-lanceolate leaves, a cylindric spike and a terete scape), which we know today as Plantago media.

Linnaeus' systematic innovation was unique and revolutionary. For an authoritative overview, see evolutionary biologist Ernst Mayr's The Growth of Biological Thought. μηδείς (talk) 18:16, 6 May 2015 (UTC)[reply]

And the answer would still be "no" – Aorun zhaoi and Guanlong wucaii spring to mind but I'm sure there are thousands of others. Mogism (talk) 00:09, 6 May 2015 (UTC)[reply]

Notice, zhaoi and wucaii are not simple Mandarin, but Latinized forms meaning Zhao's and Wucai's. μηδείς (talk) 00:33, 6 May 2015 (UTC)[reply]

To answer Nimur's question, the ICZN is the body responsible for (animal) nomenclature. Tevildo (talk) 00:29, 6 May 2015 (UTC)[reply]

Thank you, Tevildo. Nimur (talk) 01:21, 6 May 2015 (UTC)[reply]

MOID

I've been wondering for a while, for impact probability calculating purposes, how one finds Minimum Orbital Intersection Distance between two objects, using their Keplerian Orbital elements. I'm trying to find possible impacts, close approaches, and flybys between asteroids in the asteroid belt and Kuiper belt, but have come across difficulty finding anything that would explain it. Additionally, would it be possible to calculate MOID taking into account actual locations objects have relative to one another? (e.g. Jupiter Trojans cross the orbit of Jupiter frequently, and could theoretically could pass extremely close to Jupiter, but never do due to their locations 60 degrees ahead and behind of the planet.) I know it's a bit of a complicated question, but it's been confusing me for quite a while. exoplanetaryscience (talk) 23:59, 5 May 2015 (UTC)[reply]

Such problems can be solved numerically using a minimization algorithm (an application of numerical optimization) or analytically, using methods of the calculus of variations. Consider the analytical solution with a grain of salt, though, because the orbit parameters are derived numerically...

Nimur (talk) 00:21, 6 May 2015 (UTC)[reply]

Nimur's grain of salt: Orbital dynamics of that many objects are subject to the issues of the n-body problem, which states that for any number where n>2, the system behaves chaotically, and the individual positions of the respective bodies cannot be modeled reliably on long enough time frames. Predicting the interactions between individual objects in such belts is impossible, quite literally. General trends could be predicted (such as one may expect X number of impacts in Y time frame), but one could not say that any specific object would reliably collide with any other, in any sufficiently long time frame. --Jayron 32 01:58, 6 May 2015 (UTC)[reply]